The United States has reacted to Russia’s announcement that its fighter jets stopped using an airbase in western Iran to target terrorists in Syria.

The US State Department on Monday said that it was unclear if Moscow's use of the airbase had "definitively stopped."

"We're monitoring closely, we continue to. It's not clear to us other than what we've seen in various press and public statements whether their use of this airbase has definitively stopped, but we'll continue to watch it closely,” State Department Spokesman Mark Toner said.

Toner stated that the Russian-Iranian cooperation was part of the "alarming" situation in Syria, but then added, “I'm not aware that we have information one way or the other what's going on here.”

US State Department Spokesman Mark Toner

Earlier on Monday, Russia's ambassador to Iran said all of his country’s aircraft had left the Nojeh Airbase near the western Iranian city of Hamedan.

However, the Russian Defense Ministry said in a separate statement that Moscow could use the Iranian airbase again depending on the situation in Syria and as long as Iran agreed.

Iran's Foreign Ministry spokesman also confirmed the end of Russia’s use of Nojeh Aairbase, saying the move was part of an operational cooperation between Tehran and Moscow.

“Russia has no base in Iran and is not stationed here. They did this (operation) and it is finished for now," Bahram Qassemi told reporters.

Spokesman for the Iranian Foreign Ministry Bahram Qassemi speaks during his first press conference in Tehran, Iran, August 22, 2016.

Iranian Defense Minister Brigadier General Hossein Dehqan had said that Russia would be allowed to use the Nojeh base "for as long as they need.”

When it was reported last week that Russia was using the Iranian airbase, the US State Department called the move "unfortunate but not surprising" and said that it was examining whether that was a violation of a United Nations resolution.

Iran and Russia are Syria’s main allies and have been assisting the government of President Bashar al-Assad in its fight against foreign-backed terrorist groups, including Daesh terrorists.

Since September 30, 2015, Russia has been conducting airstrikes against Daesh and other terrorist groups in Syria at the Syrian government’s request.

Iran has also been providing advisory assistance to the Syrian government.